import java.util.LinkedList;
import java.util.Queue;

//被围绕的区域
//测试连接： https://leetcode.cn/problems/surrounded-regions/
public class Solve {
    int[] dx = { 0, 0, 1, -1 };
    int[] dy = { 1, -1, 0, 0 };
    int m, n;

    public void solve(char[][] board) {
        m = board.length;
        n = board[0].length;

        // 1. 先处理边界的 'O' 联通块，全部修改成 '.'
        for (int j = 0; j < n; j++) {
            if (board[0][j] == 'O')
                bfs(board, 0, j);
            if (board[m - 1][j] == 'O')
                bfs(board, m - 1, j);
        }
        for (int i = 0; i < m; i++) {
            if (board[i][0] == 'O')
                bfs(board, i, 0);
            if (board[i][n - 1] == 'O')
                bfs(board, i, n - 1);
        }
        // 2. 还原
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                if (board[i][j] == 'O')
                    board[i][j] = 'X';
                else if (board[i][j] == '.')
                    board[i][j] = 'O';
    }

    public void bfs(char[][] board, int i, int j) {
        Queue<int[]> q = new LinkedList<>();
        q.add(new int[] { i, j });
        board[i][j] = '.';
        while (!q.isEmpty()) {
            int[] t = q.poll();
            int a = t[0], b = t[1];
            for (int k = 0; k < 4; k++) {
                int x = a + dx[k], y = b + dy[k];
                if (x >= 0 && x < m && y >= 0 && y < n && board[x][y] == 'O') {
                    board[x][y] = '.';
                    q.add(new int[] { x, y });
                }
            }
        }
    }
}
